fix(session): roll back canceled pending creates

[julianknutsen]

Summary

• Roll back pending-create session beads when an async start succeeds but the controller context is canceled before commit.
• Avoid stamping city-stop sleep metadata onto creating sessions; create rollback owns that state.

Tests

• go test ./cmd/gc -run 'TestMarkCityStopSessionSleepReasonSkipsCreatingSessions|TestCommitAsyncStartResultWithContext_(SkipsCanceledCommit|StopsCanceledSuccessfulPendingCreateRuntime|RollsBackCanceledPendingCreateSuccess|RollsBackCanceledPendingCreateError)' -count=1
• pre-commit hook: go test -json -p=4 -count=1 ./...

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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